Concentric ring tracks are defined in the magnetic medium on disks. Data may be recorded on the tracks or previously recorded data may be read from the tracks. The width of a read-write head is coextensive with the spacing between adjacent tracks. With the read write head aligned with the centerline of a track, the read-write head extends half way to each adjacent track. A positioning circuit controls a read-write head servo drive mechanism to position the read-write head over a selected track. During a write operation, the read-write head must be precisely aligned with the selected track so that data written onto the disk is properly positioned on the disk relative to the track. During a read operation, the read-write head must also be precisely aligned with the selected track so that data read from the track is interpreted correctly. To assure precise alignment of the read-write head with the selected track, each track in an embedded servo implementation is segmented to include servo fields, with segments between the servo fields used for recording data. The servo field includes read-write head positioning information, which when read and demodulated, can be used to ascertain whether the read-write head is precisely aligned with a selected track or whether the read-write head needs to be repositioned to be precisely aligned with the selected track.
One technique used to evaluate alignment of a read-write head with the center of the track is a time division multiplexed technique. The relative amplitude of the read position field signals are employed to determine the position of the read-write head relative to a track. Up to four position fields are typical. The signal produced by the read-write head when reading the position fields are in the form of bursts. When four position fields are present they are often designated A, B, C, and D. Position fields A, B, C and D are located on each track at spaced intervals around the track. When the read-write head is aligned with the track, that is, aligned with the center line of the track, the A and B fields are read at half amplitude strength, the C field is read at full amplitude strength, and the D field is read at zero amplitude strength. A servo demodulator circuit is employed to derive the appropriate control action from the position field bursts.
Servo demodulator circuits are typically one of two types. The first type of demodulator circuit utilizes a peak detecting circuit. The peak detecting circuit compares the relative magnitudes of the amplitude of the A, B, C and D position field signals to determine whether the read-write head is aligned with the track. The second type of demodulator circuit is an integrating technique that demodulates position errors for improved linearity and reduced sensitivity to irregularities in the disk surface. These demodulator circuits have limited linearity that is inadequate for higher frequency operation.
It would be desirable to have a demodulator circuit that would accommodate wide variations in the signal received from the read-write head. Such a demodulator circuit would normalize the signal received from the read-write head to compensate for amplitude variations and the number of pulses per burst and simultaneously maintain speed and linearity.